A charged particle beam apparatus, which is disclosed in JP-A-5-40479 (1993), will be explained with reference to FIG. 1.
In FIG. 1, the charged particle beam 82 is directed along a path extending in the direction Z. When sine wave currents, having a phase difference of 90.degree. with respect to each other, are supplied to a scanning electromagnet 80 to produce a magnetic field in the X direction and to a scanning electromagnet 81 to produce a magnetic field in the Y direction, respectively, the charged particle beam is scanned circularly by the magnetic fields generated by the respective electromagnets. When the circularly scanned charged particle beam is projected onto a scatterer 83, the size of the charged particle beam is broadened, and the dose in the irradiation region is distributed as shown in FIG. 2. The dose becomes uniform in the region within a range of 2r, but the dose in the region outside the range 2r decreases more for positions farther from the central position and becomes uneven. Accordingly, the uneven irradiation region was cut by a collimator, and only the irradiation region having a uniform dose was used for irradiating an affected portion.
The patent JP-A-7-275381 discloses a method, by which electromagnets are controlled so that the irradiation region is formed in an arbitrary shape.